394 



ORGAN SYSTEMS OF MAN 



and more intricate until a telephone system 

 like that of New York City is about as diffi- 

 cult to understand as the nervous system of 

 a grasshopper or a man. 



Among some of the higher invertebrates 

 and all the vertebrates the network of tiny 

 fibers connecting all parts of the animal has 

 apparently proven inadequate, because a 

 supplementary system has evolved, namely, 

 an endocrine system. In this system an 

 entirely different principle is employed; in- 

 stead of impulses passing over tiny fibers, 

 specific chemicals produced by special 

 glands are released into the blood and cir- 

 culate to other parts of the body where they 

 produce a specific effect. This method has 

 proven very satisfactory for certain types of 

 responses, as will be pointed out a little 

 later in this chapter. 



In handling this very complex problem 

 of coordination we shall use man as our 

 example. We shall begin with a discussion 

 of stimuli from the external world and the 

 internal environment as received by the 

 sense organs and other receptors, then pro- 

 ceed to the nervous system which is the 

 intermediary for the transmission and inter- 

 pretation of the stimuli, and conclude with 

 the eflFectors — muscles and glands — which 

 give the response. 



RECEPTORS 



The receptors in the human body consist 

 of specialized end organs located in stra- 

 tegic regions and are highly sensitive to 

 certain kinds of stimuli. Conspicuous sense 

 organs such as the eye and ear are familiar 

 to everyone; others, such as the tiny recep- 

 tors located in the muscles and other parts 

 of the internal body, are not so well known 

 but are just as important in the proper co- 

 ordination of the organism. 



Skin receptors 



It might be expected that the outer cov- 

 ering of the body would be highly sensitive 

 to the environment around it, and this is 



true, from the lowly planarian to man him- 

 self. A pin prick in the skin almost any- 

 where over the surface of the body results 

 in a pain sensation; this fact indicates that 

 these nerve endings are very numerous and 

 widespread. The same is true of the nerve 

 endings for touch, pressure, heat, and cold. 

 A thin section of the human skin will reveal 

 tiny, oval-shaped tactile corpuscles from 

 which nerves lead inward. Any pressure 

 brought to bear on them causes impulses 

 to be discharged from the specialized cells 

 within the corpuscle which travel along 

 nerve fibers to the central nervous system. 

 Other kinds of sensory end organs which 

 respond to pressure stimuli over larger 

 areas are located in the deeper skin and in 

 many internal organs. Free nerve endings 

 which register pain terminate in the epi- 

 thelium within the internal organs as well 

 as in the skin. The endings ramify and come 

 into contact with nearly every cell, which 

 explains why pain sensations are felt even 

 if only a small area is stimulated, such as 

 in pricking with a pin. 



By marking off specified areas on the skin 

 and using a stiff bristle as a stimulus the 

 appropriate receptors can be located, and 

 they will be found to be quite unevenly dis- 

 tributed over the body. It is difficult, for 

 example, to distinguish two points one-half 

 inch apart in the middle of the back, 

 whereas on the tip of the finger or tongue 

 a distance of one-sixteenth of an inch is 

 perceptible. Likewise, if metal pointed in- 

 struments (styluses) are used, the hot and 

 cold end organs can be detected. There are 

 more cold spots than hot spots and that is 

 why, for instance, one shivers at first if sud- 

 denly exposed to a hot shower. When all of 

 the end organs are stimulated simultane- 

 ously, as would be the case in the above 

 situation, the total response is that of cold- 

 ness at first because there are more of the 

 cold than hot spots. Later, the proper in- 

 terpretation of the stimulation is recog- 

 nized. Pressure end organs can be found by 

 applying a blunt metal stylus having the 



